1. Field
Exemplary embodiments relate to a burner to heat a fuel reformer that generates hydrogen, and more particularly, to a fuel reformer burner in which fuel gas and anode-off-gas (AOG) that are not used in a fuel cell stack and that pass through the fuel cell stack may be burned so that combustion efficiency is enhanced.
2. Description of the Related Art
Fuel cells are power generation systems which directly convert chemical energy into electrical energy through a chemical reaction between hydrogen and oxygen. Hydrogen is contained in a hydrocarbon-based material, such as methanol, ethanol, or natural gas.
Polymer electrolyte membrane fuel cell (PEMFC) systems are high-efficiency next-generation power generation systems which produce electricity and heat through an electrochemical reaction between hydrogen and air. Such fuel cell systems include a fuel cell stack and a fuel processing unit as main elements and a fuel tank and a fuel pump as auxiliary elements. The fuel cell stack has a configuration in which several to several tens of unit cells, each unit cell including a membrane electrode assembly (MEA) and a separator, are stacked. The fuel processing unit includes a fuel reformer, a shift reactor, and a carbon monoxide (CO) remover.
Hydrogen generated in the fuel processing unit is supplied to anodes of a PEMFC stack and reacts with oxygen supplied to a cathode to produce electricity.
A reforming reaction in a fuel reformer is performed at a high temperature. Thus, the fuel processing unit includes a fuel reformer burner to supply heat to the fuel reformer.
The fuel reformer burner burns fuel gas and produces heat. In this case, the fuel gas mainly consists of hydrocarbon gas, such as methane gas. However, in order to improve the efficiency of the PEMFC systems, the fuel reformer burner burns not only fuel gas, such as hydrocarbons, but also hydrogen gas. In other words, a unit for burning the hydrogen gas contained in an anode-off-gas (AOG) is provided because generally about 70% to about 85% of the hydrogen gas is reacted in the PEMFC stack, and the hydrogen fuel that is not reacted within the PEMFC stack is discharged out of the PEMFC stack and is discarded. Thus, when the hydrogen fuel is recovered and used as fuel in a burner for the fuel processing unit, the efficiency of the entire power generation system may be increased.
In addition, when the fuel processing unit is started, a large amount of CO is contained in hydrogen gas for several tens of minutes until the fuel processing unit reaches a normal operating state, thus this hydrogen gas is not supplied to the PEMFC stack. This is because a large amount of CO acts as poison to an MEA catalyst. As a method of utilizing hydrogen gas having a high concentration of CO, the hydrogen gas may be used as fuel gas for the fuel reformer burner. This helps reducing energy consumption when a fuel cell operates.
Thus, in order to enhance the efficiency of fuel cell systems, there is a need for a burner in which hydrogen gas contained in the AOG as well as fuel gas such as hydrocarbon is burned. To this end, there is a need for a burner in which both types of fuel (fuel gas and hydrogen gas) may be efficiently and safely burned.
In the related art, air and AOG are simultaneously mixed in a fuel inlet of the fuel reformer burner. However, the flame speed of hydrogen during combustion is very high compared to the flame speed of the fuel gas, i.e., about ten times faster than that of methane. Thus, the flame may travel upstream or opposite to a direction in which a fuel flows, which may cause an explosion.